Underground conduits are widely used for the transmission of fluids, such as in pipelines and the like, as well as for carrying wires and cables for the transmission of electrical power and electrical communication signals. While the installation of such conduits is time-consuming and costly for locations where the earth can be excavated from the surface, the routing of such conduits becomes more difficult where such surface excavation cannot be done due to the presence of surface obstacles through which the excavation cannot easily proceed. Such surface obstacles include highways and railroads, where the installation of a crossing conduit would require the shutdown of traffic during the excavation and installation. Such surface obstacles also include rivers, which present extremely difficult problems for installing a crossing conduit, due to their size and the difficulty of excavation thereunder.
Prior methods for the installation of conduits have included the use of directional drilling for the formation of an inverted underground arcuate path extending between two surface locations and under the surface obstacle, with the conduit installed along the drilled path. A conventional and useful method for installing such underground conduits is disclosed in U.S. Pat. No 4,679,637, issued Jul. 14, 1987, assigned to Cherrington Corporation, and incorporated herein by this reference. This patent discloses a method for forming an enlarged arcuate bore and installing a conduit therein, beginning with the directional drilling of a pilot hole between the surface locations and under a surface obstacle such as a river. Following the drilling of the pilot hole, a reamer is pulled with the pilot drill string from the exit opening toward the entry opening, in order to enlarge the pilot hole to a size which will accept the conduit, or production casing in the case of a pipeline conduit. The conduit may be installed during the reaming operation, by the connection of a swivel behind the reamer and the connection of the conduit to the swivel, so that the conduit is installed as the reaming of the hole is performed. Alternatively, the conduit can be installed in a separate operation, following the reaming of the pilot hole (such reaming referred to as "pre-reaming" of the hole). Additional examples of the reaming operation, both as pre-reaming and in conjunction with the simultaneous installation of the product conduit, are described in U.S. Pat. No. 4,784,230, issued Nov. 15, 1988, assigned to Cherrington Corporation and incorporated by this reference.
While the above-described methods are generally successful in the installation of such conduit, certain problems have been observed, especially where certain types of sub-surface formations are encountered. Referring now to FIGS. 1 and 2, examples of such problems in the installation of conduit in an underground arcuate path will now be described.
FIG. 1 illustrates the reaming operation described above, in conjunction with the installation of production conduit as the reamer is pulled back. In the example of FIG. 1, entry opening 0 is at surface S on one side of river R; exit opening E is on the other side of river R from entry opening 0. At the point in the installation process illustrated in FIG. 1, a drilling apparatus, including a hydraulic motor 14 mounted on a carriage 16 which is in place on an inclined ramp 12, has drilled the pilot bore hole B from entry 0 to exit E, using drill string 10, and the reaming and installation is in progress. Motor 14 is now pulling reamer 48, to which production conduit 46 is mounted, back from exit E toward entry 0. Reamer 48 is larger in diameter than the diameter of production conduit 46. Upon completion of the reaming operation of FIG. 1, if successful, production conduit 46 will be in place under river R, and extending between exit E and entry 0.
Referring now to FIG. 2, a close-up view of the location of reamer 48 and production conduit 46 in FIG. 1 is now illustrated. Leading drill string section 10C is attached by way of tool joint 52 to reamer 48, reamer 48 having cutting teeth at its face. Swivel 50 connects production conduit 46 to reamer 48, by way of extension 62 connected to a sleeve 66 on conduit 46. As is evident from FIGS. 1 and 2, bore hole B is enlarged to enlarged opening D by operation of reamer 48. Conventional sizes of conduit 46 are on the order of 20 to 48 inches in outside diameter, with the size of reamer 48 greater in diameter than conduit 46. Due to reamer 48 being larger than conduit 46, an annulus 68 surrounds conduit 46 as it is pulled into the hole D. Provision of the annulus 68 allows for reduced friction as the conduit 46 is placed therein.
As noted above, prior techniques have also included a pre-reaming step, wherein a reamer, such as reamer 48, is pulled back from exit E to entry 0 without also pulling production conduit 46 into the reamed hole. In such a pre-reaming step, a following pipe generally trails reamer 48 in such the same manner as conduit 46 trails reamer 48 in FIGS. 1 and 2, to provide a string for later installation of conduit 46. Such a trailing pipe will be of a much smaller size than conduit 46 of FIGS. 1 and 2, for example on the order of five to ten inches in diameter.
It has been observed in the field that both the prereaming and reaming with installation operations are subject to conduit or pipe sticking problems, especially as the size of the production conduit increases in diameter, and as the length of the path from entry 0 to exit E increases. Such sticking is believed to be due, in large degree, to the inability to remove cuttings resulting from the reaming operation. Due to the large volume of earth which is cut by way of the reaming operation, and the generally low fluid flow velocity of drilling or lubricating mud or fluid into the reaming location, the velocity of cuttings circulating from the reaming location is minimal. While the mud or other lubricating fluid flow could be increased in order to increase the velocity of the cuttings from the reaming location, such an increase in the velocity of the fluid could result in such undesired results as hole wall erosion and fracturing through the formation.
Due to the inability to sufficiently remove the cuttings during the reaming operation, it is believed that the cuttings pack together near the location of the reamer. Many of the cuttings from the reaming operation are heavier than the fluid transporting them and, in such large diameter holes as are required for the installation of conduit, these large cuttings will fall out or settle toward the bottom of the hole first, and then build up into a circumferential packed mass, causing a poor rate of reaming. Referring to FIG. 2, where a production conduit 46 is being pulled through with reamer 48, it is believed that such packing will begin at locations P surrounding the leading end of conduit 46, and also along the sides of conduit 46 in annulus 68. As the cuttings pack together, squeezing whatever water or fluid is present therein, the density of the packed mass increases. Upon sufficient packing, it is believed that pressure builds up ahead of locations P, toward the bit of reamer 48, such pressure resulting from the mud or fluid continuing to be pumped into the reaming location with the return flow reduced at locations P around conduit 46 in annulus 68. It is also believed that this buildup of pressure will also force cuttings into bore hole B ahead of reamer 48, and that these cuttings will also begin to pack, most likely at locations P' near the first tool joint 70 ahead of reamer 48.
The buildup of pressure between locations P and P' surrounding reamer 48 causes significant problems in the reaming operation. Such effects have been observed in the field during reaming operations, when the reamer cannot be rotated, pulled or pushed at a particular location in the operation. It should be noted that the sticking of the reamer occurs both for the pre-reaming operation described hereinabove and for the combined reaming and pulling operation. It should further be noted that the pressure buildup described hereinabove is believed to be worse in high pressure formations such as clay.
Another undesired effect resulting from the buildup of pressure when the reamer cuttings are insufficiently removed is similar in nature to differential sticking in the downhole drilling field. As is well known in the downhole drilling art, differential sticking of the drill string occurs when the pressure of the drilling mud surrounding the drill string is greater than the pressure exerted by the surrounding formation. In the event that the caking of drilling mud and the structure of the well bore is not strong enough to maintain its shape when presented with such a differential pressure, the pressure of the drilling mud can force the drill string into the formation, holding it there with sufficient pressure that it cannot be released from the surface.
It is now believed that similar effects can be present in the field of installation of underground conduit, due to insufficient removal of the reaming cuttings. If the pressure near reamer 48, when packed off as described hereinabove, is sufficiently greater than the pressure exerted by a surrounding formation, the conduit 46 can be driven into the formation, causing sticking of the conduit 46 thereat. It should be noted that the installation of underground conduit is particularly susceptible to such sticking, since much of the formations underlying rivers are sedimentary or alluvial formations, with relatively thin layers of differing strength. Accordingly, the drilling and reaming operations in river crossing installations are exposed to many differing formations along the length of the path, with the likelihood of encountering a weak (in pressure) formation being relatively large. Accordingly, such pressure buildup due to insufficient reaming cutting removal can cause conduit sticking at particular locations along the underground path.
Furthermore, it should be noted that the insufficient removal of cuttings impacts the reaming operation itself. If cuttings are not sufficiently removed from the reaming location, a number of cuttings will tend to be present in front of reamer 48 of FIG. 2; as a result, reamer 48 will tend to recut its own cuttings, rather than cutting the earth in its path and enlarging the hole. This results in poor penetration rates for the reaming operation. As noted above, as the reaming rate slows, the pressure buildup between the packed locations will accelerate, further degrading the operation and increasing the likelihood of the reamer and conduit sticking.
In addition, the recutting of the cuttings results in a high degree of reamer wear, both at the teeth and also in the parent metal of reamer 48. In rotor reamers, such wear has been observed also at the seals and bearings. This has also been observed for reamers which use carbide-coated rotating cones as the cutting bits, in similar manner as a downhole tri-cone bit; while the carbide wears slowly, the insufficient removal of the cuttings has been evidence in significant wear of the parent metal of the reamer. Furthermore, as the cuttings become smaller due to multiple recutting cycles, the cuttings which are removed with the drilling mud are much more difficult to process by the solids control system.
Other methods for installing conduit in an underground path includes forward thrust techniques, such as described in U.S. Pat. Nos. 4,176,985, 4,221,503 and 4,121,673. Particularly, U.S. Pat. No. 4,176,985 discloses an apparatus which thrusts a casing into a pilot hole, with a bit leading the casing. However, while such forward thrust techniques are useful for unidirectional application such as the introduction of conduits into the ocean, such methods place significant stress on the conduit itself, and also present relatively slow installation rates. The pull-back methods described hereinabove and hereinbelow are preferable from the standpoint of reduced stress on the casing, as well as increased installation rates.
A method and apparatus for removing cuttings is described in U.S. Pat. No. 5,096,002 to Cherrington, filed Jul. 26, 1990, entitled "Method and Apparatus for Enlarging an Underground Path", which is incorporated by reference herein. While the device described in U.S. Pat. No. 5,096,002 is effective in removing the cuttings, it relies on several moving parts, which may decrease its reliability.
Therefore, a need has arisen in the industry for a method and apparatus for removing cuttings from a bore hole with a reduced number of working parts.